Waveguide filter



Sep-t. 23, 1 958 J. EPsTElN lWAVEGUIDE FILTER 2 Sheets-Sheet 1 I Filed Dec. 1. 1954 High F 4 fz o y mm m mm m mf E JW/ Sept. 23, 1958v J. EPsTElN WAVEGUIDE FILTER Filed Dec. l, 1954 2 Sheets-Sheet 2 NZZ 74 f6,

wf" m w, 762. 52u/ ya mi* 10170 INVENTOR. .IESS EP.: rE/N United States Patent WAVEGUIDE FILTER Jess Epstein, Princeton, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application December 1, 1954, Serial No. 472,317

19 Claims. (Cl. S33-6) The present invention relates to electromagnetic wave filters and in particular to improved waveguide filters.

It is an object of the present invention to provide an improved waveguide lter operative in the ultra high frequency and microwave regions.

It is another object of the present invention to provide an improved waveguide filter of extremely high power handling capacity.

It is still another object of the present invention to provide an improved vestigial sideband filter especially suitable for use in a television transmission system.

Still another object of the present invention is to provide an improved vestigial sideband filter of much simpler structure than existing vestigial sideband filters.

Yet another object of the present invention isto provide an improved waveguide filter having constant input resistance.

A more specific object of the present invention is to provide an improved waveguide filteradapted to couple two separate generators of different frequencies to a common load such as, for example, both a picture frequency generator and a sound frequency generator to a television system radiating means.

A typical embodiment of the present invention includes a main waveguide dimensioned to support the propagation of a given mode of linearly polarized wave energy in a band of frequencies. ,This band includes certain desired frequency components and certain undesired frequency components. Positioned along the length of the main waveguide are a plurality of cavity resonators, each coupled to the waveguide and each tuned to a different, undesired frequency. The entire filter is series resonant at the desired frequency. Located between the input means and the cavity resonators is a circular polarizer, whereby linearly polarized energy travelling from the input means toward the cavity resonators is converted to circularly polarized wave energy. The circularly polarize'd'wave energy at the undesired or reject frequencies is refiected back from the cavity resonators and reconverted to linearly polarized wave' energy by the circular polarizer. This reflected, linearly'polarized energy is in space quadrature with the input linearly polarized wave energy' and may be extracted from the waveguide by a suitably positioned device such as a probe extending into the guide. In one form of the invention, the probe leads to a dissipating load which substantially fully 'attenuates energy at the undesired frequencies.

The energy at the desired frequency passes through the waveguide substantially unaffected by the cavity resonators. Beyond the cavity resonators a second circular polarizer recouverts this circularly polarized energy to linearly polarized energy. An output means including a suitably positioned probe located at the output end of the waveguide extracts Vthis energy from the waveguide and passes it to an output load such as an antenna or the like. v

In one form of the invention the main. waveguide may end thereof.

ice

be circular and the cavity resonators cylindrical and symmertically disposed about the circular waveguide. In another form of the invention the main waveguide may be rectangular and the cavity resonators also rectangular. The input and output means may include coaxial lines, waveguides or broad band transition means between the input and output transmission lines and the main waveguide.

Another embodiment of the invention is especially adapted for connecting two different generators to aA common load. One of the generators is coupled` to the main waveguide at one end thereof and the other at the other As in the embodiment above described, one or more cavity resonators located along the length of the guide (between the input and output end thereof) may be' tuned to certain reject frequencies in the band associated with the first of the generators. Another of the cavity resonators may be tuned to a second input frequency associated with the other generator. A circular polarizer is located between each generator input and the cavity resonators.

The desired frequency components produced by the first generator are transmitted down the guide and changed from linearly to circularly polarized' wavesand then from circularly polarized waves to linearly polarized waves. An output probe connected to a common output load picks up this output energy. The undesired frequency components produced by the first generator are reflected back from the cavity resonators to an absorbing load as in the embodiment discussed` above.-`

The input wave applied by the second generator is changed to a circularly' polarized wave, reflected back from the cavity tuned to its frequency and reconverted to a linearly polarized wave travelling in the same direction and having the same plane of polarization as the wave of desired frequency components from the first generator. lt is therefore also applied to the common load. The undesired frequency components, if any, of the second wave are passed through the waveguide tothe other input end thereof and attenuated by theabs'orbing load.

The invention will `be describedv in' greater detail by" reference to the following description-taken in` connection with the following drawing' in which:

Fig. l is a view, partially infcross-section, ofa typical embodiment of'the present invention;

Fig. 2 is a cross-sectional `view along line 2-2` of Fig. 1;

Fig. 3 is a cross-sectional View along line 3 3; of Fig. l;

Fig. 4 is argraph of the performance ofthe lter of Fig. 5 is a plan view of a second embodiment of the? invention;

Fig. 6 is a cross-sectional view along line 6-6 of Fig. 5;

Fig. 7 is a View, partially .in cross-section, of aV third embodiment of'th'e invention;l

Fig. 8 is a cross-sectional view along line 8 8 of Fig. 7; and

Fig. 9 is a plan view of a modified input coupling means usable with the embodiments of Figs. 14 and 5".

Throughout the figures similar reference numerals The end of coaxial line;

a wave-lengths, at the frequency it is desired to transmit down the waveguide, from'the input end 16 of the waveguide. Thus, looking toward the input end 16 from input probe 14 the input end apepars as an open circuit and substantially all wave energy is transmitted to the right.

Circular polaizing means 18, 18 comprise a pair of quarter wave ns tapered at the ends 19, 19 thereof for matching purposes. These convert the linearly polarized wave travelling from the input means into a circularly polarized wave. Although shown as a pair of fins, it will be appreciated that other circular polarizing devices such as metal or'dielectric rods or a dielectric slab may be used instead. The theory of operation of circular polarizing devices is well known and need not be discussed in detail here.

Located along the axial length of circular waveguide 12 are a plurality of cavity resonators 22, 24, 26, each coupled to the waveguide by means of an annular coupling aperture one of which, 28, is shown. These cavity resonators are tuned to different frequencies in the input frequency band which it is desired to reject. The susceptance slope of the cavities may be varied by adjusting cavity dimensions and the widths of the annular coupling apertures. The filter is made series resonant for a desired frequency in the input band by means of a plurality of transformers, one located in front of each cavity resonator.

-Each cavity resonator may be provided with a plurality of equiangularly spaced tuning slugs located in a side wall of the cavity resonator. for tuning out electrical and mechanical dssymmetries in the resonators. For the sake of drawing simplicity these are not illustrated.

Referring again to Fig. 1, the input linearly polarized wave is converted to a circularly polarized wave by polarizer 18. Cavity resonators 22, 24 and 26 are tuned to reject frequencies f1, f2 and f3 respectively, in the input frequency band. These resonators therefore appear as open circuits to these frequencies and the energy at these frequencies is reflected back from the cavity resonators toward circular polarizers 18, 18. The circular polarizer recouverts the circularly polarized waves at frequencies f1, f2, f3 to linearly polarized waves having a direction of polarization in space quadrature with that of the input wave. i

Coaxial line 34 is electrically coupled to waveguide 12 by means of coupling probe 36 which is at an angle of 45 with respect to the common axis of fins 18, 18. Preferably probe 36 is spaced an odd number of guide quarter wavelengths from the short-circuited end 16 of the waveguide at approximately the average reject frequency. Thus, the wave energy at the rejected 'frequencies looking from coupling probe 36 toward shortcircuited end 16 sees an open circuit and is substantially fully transmitted into the coaxial transmission line 34.

In a preferred form of the invention, line 34 may be terminated in anabsorbing load 38 made of Bakelite, ferrous metal or some other energy dissipating means for substantially fully dissipating the rejected wave energy. If, however, it is desired to utilize the rejected frequencies, the coaxial line 34 may be terminated in a utilization device.

As already mentioned, the lter is series resonant at a frequency of interest in the input band. Energy at this frequency is converted from linearly to circularly polarized wave energy by circular polarizer 18, 18'. Cavity resonators 22, 24 and 26 are tuned to frequencies 4lower than the frequency of interest and have substantially vlittle effect on this frequency. Located in the waveguide near the output end thereof is a second circular polarizer 40, 40. Again, although shown as a pair of tapered ns, the polarizer may consist of other types of polarizing devices well known in the art. second circular polarizer converts the circularly polarized Two such transformers 30, 32 are shown.

This,

4 wave energy at the frequency of interest to linearly polarized waves having a direction of polarization which depends upon the angular position of tins 40,'40'.

The output means includes a coupling probe 42 which may be an extension of the inner conductor of coaxial line 44. The output coupling probe is spaced an odd number of guide quarter wavelengths, at the frequency of interest, from the remote end 47 of the waveguide. Therefore, looking toward the remote end of the waveguide from coupling probe 42 the former appears as an open circuit and substantially all energy at the frequency of interest passes into coaxial transmission line 44 and thence to the output load 46.

Fig. 4 is a graph of the performance of the lter -system illustrated in Figs. 1-3. The curve dips at frequencies f1, f2, and f3. In an embodiment of the invention actually constructed, the attenuation at the reject frequencies was greater than 50 db with respect to the frequency of interest, as clearly illustrated on the graph.

In the embodiment ofthe invention discussedv above, the input and output means include coaxial transmission lines. It is to be understood thatA this is merely illustrative of the invention and is not meant to be limiting. Other input and output means such as waveguides may be employed and, if desired, broad band coupling means may be employed between the main waveguide and, the input and output means. If it is desired to substitute waveguides for the coaxial lines, these would be coupled to the circular guide with their broad dimension parallel to the waveguide axis. One typical broad band matching device which may be employed instead of the Iinput connection shown in Figs. 1-3 is illustrated in Fig. 9. This includes an input coaxial line 10 coupled to a rectangular waveguide 50 which is in turn coupled to the circular waveguide 12. The coaxial line 34 leading to the absorbing load is shown by dashed lines. This line is spaced an odd number of guide quarter wavelengths from the short-circuiting end 16 of the waveguide.

Although in the embodiment of the invention discussed above three cavity resonators are employed, it is to be understood that the invention is not limited to the use of three resonators. For certain purposes a singlel cavity may be sufficient and for others it may be desirable to use two, four or more cavity resonators.

A second embodiment of the invention is shown in Fig. 5.- This one is particularly adapted for coupling two generators having different carrier frequencies to a single load. For example, it is especially suitable for coupling the picture and sound transmitters of a television transmitting station to a single antenna. The input wave energy applied to coaxial line 10 includes the picture frequency fp and certain other, lower frequencies it is desired to filter out of the system. Cavity resonators 22 and 24 are tuned to these undesired frequencies and, as in the embodiment of Fig. l, wave'energy at the undesired or reject frequencies is reflected back toward the input end 16 of the waveguide and substantially fully transmitted to the absorbing load connected to coaxial line 34. The main waveguide 12 is series resonant at the picture frequency. 'The linearly polarized input wave energy at the picture frequency is converted to circularly polarized wave energy by polarizer 18, 18 (only 18 is shown). This circularly polarized wave energy travels down the waveguide and is reconverted to linearly polarized wave energy by circular polarizer 40, 40. Output coupling probe 42 is at an angle of 45 with polarizer 40, 40' and receives the energy at the picture frequency.

Wave energy yat the sound frequency fs is applied to the second input coaxial line `60. This line is coupled to waveguide 12 by means of coupling probe 62 (Fig. 6)

f positioned at an angle Vof 45 to the plane passing through 40, 40 and in space quadrature with output probe 42;

Cavity resonator 26` is tuned to the sound frequency and '5 therefore looks like an open cir-cuit in series with the waveguide at the sound frequency.

In operation, input wave energy at the sound frequency is converted by circular polarizer 40, 40 to circularly polarized wave energy. The circularly polarized wave energy is reflected back from cavity resonator 26 tow-ard the remote end 47 of the waveguideand polarizer 40, 40 reconvents Ithe circularly polarized wave energy to linearly polarized wave energy. The reflected, linearly polarized wave energy is in space quadrature with the incident, linearly polarized wave energy at the sound frequency. Therefore, output probe 42 is suitably oriented to couple this reflected wave to output load 46.

For optimum performance probe 42 should be spaced an odd number of guide quarter wavelengths, at the sound frequency, from the remote end 47 of the waveguide. Also, the probe should be spaced an odd number iof guide quarter wavelengths, at the picture frequency, from the remote end 47 of the waveguide. These requiremen-ts conflict; however, if the two input frequencies are relatively close toge-ther then the two spacings are substantially the same. In an embodiment of the invention actually constructed the picture and sound frequencies -were within 1% of one another and the spacing between probe 42 `and waveguide end 47 was adjusted to a guide quarter wavelength at the picture frequency. When so adjusted, there was practically no energy loss at the sound frequency. If the two input frequencies are relatively widely spaced from one another, it would be preferable to use a broad band transition section between the output load and the circular waveguide. For example, e. transition section such as shown in Fig. 9 could be employed as could la number of other broad band devices known to those skilled in the art.

summarizing the operation of the embodiment of Fig. 5, the picture frequency supplied to input means 16 is converted to circularly polarized wave energy, Ithen converted back to linearly pola-rized energy and passed into the output load. Wave energy laccompanying the wave energy at the picture frequency which itis desired to reject is reflected back from cavity resonators 22 and 24 and passed to the absorbing load connected to line 34. Input energy at the sound frequency is supplied to coaxial line 60. Cavi-tyv resonator 26 is tuned to the sound frequency `and reflects all energy Iat this frequency back to the output load 46. Any undesired frequencies accompanying the sound frequency pass into the absorbing load connected to line 34.

As Iin the embodiments of Figs. 1 3, it is to be understood that ralthough only three cavity resonators are shown, more or less may be employed according to the filter characteristics desired. Also, it is to be understood that the cavity resonators may include means for adjusting the susceptance slope of the cavitiesand for compensating for resonator dissymmetries. These are explained in more detail -in the copending .application mentioned above.

The embodiments of Figs. 1-3, 5 and 6, employ a circular main waveguide `and annular, cylindrically shaped cavity resonators. lf desired, the main cavity resonator may be rectangula-r and the cavity resonators also may be rectangular. An embodiment of the invention employing such components is 'illustrated in Figs. 7.and 8.

Referring now to Figs. 7 .and 8, lthe lfilter includes a main square waveguide 70 and a plurality -of pairs of rectangular cavityresonators 72, 72a; 74, 74a and 76, 76a. Each cavity resonator is coupled to the waveguide by means of la slot perpendicular to the main waveguide axis, one of which, 7S, is shown in Fig. 7. As in the other embodiments of the invention, the width of the slot and cavity 4dimensions may be made adjustable in order to vary the susceptance slope of the cavities. For the sake of drawing simplicity, such means are not shown but it is to be understood that they may comprise a sleeve and set screw similar to the arrangement shown in Figs.

l and 5, and tuning plungers -mounted in; the -walls of the 4resonators or in the short-circuiting ends of the reson- `ators.

The mode of operation of the filter of Figs. 7 and 8 is substantially the same as that of Fig. l'. An input wave includ-ing a desired frequency and certain other frequencies it is desired to reject is applied to input coaxial line l0. Input probe 14 excites the waveguide in its dominant TEM) mode and the latter transmits linearly polarized wave -energy toward the cavi-ty resonators positioned along its length. Circular polarizer 80, located in opposite corners of the square waveguide convert the incident linearly polarized wave energy to circularly polarized wave energy. Wave energy at the undesired frequency components is reflected back from cavity resonators 72, 72a; 74, 74a and 76, 76a. Circular polarizer 80, 80 recouverts the reflected energy to linearly polarized energy rand it is `all substan-tially fully transmitted to the absorbing load via probe 36 and .transmission line 34. T-he filter is series resonant at the desired frequency. This desired frequency therefore passes through the waveguide being converted from linearly polarized energy to circularly polarized energy by polarizer 30, 80', and reconverted back to linearly polarized wave energy by polarizer 82, 82 and thence passed via output coaxial line 44 to :an ou-tput load (not shown).

What is claimed is: l

l. A waveguide filter comprising, in combination, a waveguide dimensioned to support the progagation of wave energy in a given frequency band; input means electrically coupled to said waveguide for exciting the propagation therein of linearly polarized wave energy in said band having a' given polarization plane; a plurality of cavity resonators electrically coupled to said waveguide and spaced from one another along the length of said waveguide, each said cavity resonator being tuned to a different frequency within said band to reflect wave energy of the frequency to which said resonator is tuned back toward said input means; circularly polarizing means located in said waveguide between said input means and said plurality of cavity resonators for converting the linearly polarized wave energy travelling from said input means to said cavity resonators to circularly polarized wave energy and reconverting the circularly polarized wave energy reflected from -said cavity resonators back toward said input means to linearly polarized wave energy having a polarization plane in space quadrature with said given polarization plane; and output means electrically coupled to said waveguide in space quadrature with said input means for receiving said reflected, linearly polarized wave energy.

2. A waveguide filter as set forth in claim l, and further including second polarizing means located in` said waveguide beyond said cavity resonators for converting the circularly polarized wave energy passed by said waveguide to linearly polarized wave energy; and second output means electrically coupled to said waveguide beyond said second polarizing means for receiving said passed, linearly polarized wave energy.

3. A waveguide filter comprising, in combination, a waveguide dimensioned to support the propagation of wave energy in a given frequency band; input means electrically coupled to said waveguide for exciting the propagation therein of linearly polarized wave energy in a given polarization plane at at least two frequencies within saidband; output means coupled to said waveguide at a given angle to said input means and spaced a predetermined distance from said input means; reflector means coupled to said waveguide betweenv said input means and said output means for reflecting wave energy at one of said frequencies and passing wave energy| at the other of said frequencies; first circular polarizing means located in said waveguide between said input means and said reflector means for converting the linearly polarized wave energy travelling from said input means to said reector means to circularly polarized wave env given polarization plane, whereby said plane polarized wave energy at said other frequency is transmitted to said output means.

4. A waveguide lter as set forth in claim 3, and further including means electrically coupled to said waveguide in the region of said input means for substantially dissipating the linearly polarized wave venergy at said one frequency.

5. A waveguide filter as set forth in claim 4, said means for dissipating said wave energy including second output means coupled to said waveguide in space quadrature with said input means, said output means comprising a dissipating load.

6. A waveguide filter as set forth in claim 3, wherein said reliector means includes a cavity resonator tuned to said one frequency.

7. A waveguide filter as set forth in claim 4, wherein said waveguide comprises a circular waveguide and said reector means comprises a hollow cylindrical cavity resonator of adjustable size tuned to said one frequency concentrically arranged about said circular waveguide.

8. A waveguide filter as set forth in claim 4, wherein saidwaveguide comprises a square waveguide and said reflector means comprises a pair of rectangular cavity resonators, each tuned to said one frequency, coupled to adjacent walls of said waveguide an-d extending radially outward therefrom.

9. A waveguide filter las set forth in claim 4, and further including a second reiiector means coupled to said waveguide between said first and second polarization means for refiecting wave energy at a third frequency and passing wave energy at a fourth frequency, both within said given frequency band; and second input means electrically coupled to'said waveguide in the region of said output means and in space quadrature with said output means for exciting the propagation along said waveguide of linearly polarized wave energy in said given polarization plane at at least said third and fourth frequencies, whereby said second polarization means converts said last-named linearly polarized wave energy into circularly polarized wave energy and reconverts the circularly polarized wave energy at said third frequency reflected from said second reflector means back toward said output means to linearly polarized wave energy having a polarization plane in space quadrature with said given polarization plane and said first polarizing means converts the circularly polarized wave energy passed by said second refiector means to linearly polarized wave energy having a polarization plane in space quadrature with said given polarization plane.

10. A waveguide filter as set forth in claim 9, wherein said first and second reflector means comprise cavity resonators, one tuned to said first frequency and the other tuned to said third frequency.

' 11. A waveguide filter as set forth in claim 10, and further including means operatively associated with said cavity resonators for adjusting the susceptance slope of said resonators.

12. A waveguide filter as set forth in claim 10, wherein said waveguide comprises a circular waveguide' and said cavity resonators comprise hollow cylindrical members concentrically arranged about said waveguide.

13. A waveguide filter as set forth in claim4, and further including means for rendering said filter series 8 resonant at said other'freque'ncy, said means including an impedance transformer-located'in said waveguide between said pair of polarizing means. y y

14. In combination, a length of waveguide, input means coupled to one end portion of said waveguide for introducing a first linearly polarized wave thereto having frequency components within a first frequency band; input means coupled to the other end portion of said waveguide for introducing a second linearly polarized wave thereto having the same direction of polarization as said first wave and having frequency components within a second frequency band; a pair of cavity resonators individually coupled to said waveguide between said two input means, one tuned to a frequency component in said first band and therother tuned to a frequency component in said second band; a pair of means for converting plane to circularly polarized waves, one located in said waveguide between said cavity resonators and one of said input means and the other located in said waveguide between said cavity resonators and the other of said input means; and an output means coupled to one end portion of said waveguide oriented to receive a linearly polarized wave having a direction of polarization in quadrature with that of the first and second waves.

l5. In the combination as set forth in claim 14, said waveguide comprising a circular waveguide and said cavity Y resonator comprising annular cavity resonators concentric with the waveguide.

16. In the combination as set forth in claim 14, said waveguide being formed with coupling apertures therein for coupling said cavity resonators to said waveguide, and further including means for adjusting the effective size of said apertures.

17. In combination, a length of waveguide, input means coupled to one end portionof said waveguide for introducing a first linearly polarized wave thereto having frequency components within a first frequency band; input means coupled to the oth-er end portion of said waveguide for introducing a second linearly polarized wave thereto having the same direction of polarization as said first lwave and having frequency components within a second frequency band; a pair of cavity resona tors individually coupled to said waveguide between said two input means, one tuned to a frequency component in said first band and the other tuned to a frequency compo-nent in said second band; a pair of means for converting plane to circularly polarized waves, one located in said waveguide between said cavity resonators and one of said input means and the other located in said waveguide between said cavity resonators and the other of said input means; and output means coupled to one end portion of said waveguideV oriented to receive a linearly polarized wave having a direction of polarization in quadrature with that of the first and second waves; and a second output means coupled to the other end portion of said waveguide oriented to receive a linearly polarized wave having a direction of polarization in quadrature with thatV of the first and second waves.

18. In the combination as set forth in claim 17, further including a dissipating load connected to one of said output means.`

19. A waveguide filter comprising, in combination, a waveguide dimensioned to support the propagation of wave energy in a given frequency band, input means electrically coupled to said waveguide for exciting the propagation therein of linearly polarized wave energy in a given polarization plane at at least two frequencies within said band, a cavity'resonator coupled to said waveguide at a predetermined distance from saidinput means for reflecting wave energy at one of said frequencies and passing wave energy at the other of said frequencies,'said cavity resonator being tuned to said one frequency, circularly polarizing means located in said waveguide between said input means and said cavity resonator for lconverting the linearly polarized wave energy traveling from said input means to said cavity resonator to circularly polarized Wave energy and for reconverting the circularly polarized Wave energy reflected from said cavity resonator back towards said input means to linearly polarized Wave energy having a polarization plane in space quadrature With said given polarization plane, and output means electrically coupled to said waveguide for receiving said reflected, linearly polarized Wave energy.

References Cited in the le of this patent UNITED STATES PATENTS Feldman Ian. 11, 1949 Lewis a Nov. 28, 1950 Purcell Aug. 19, 1952 Roberts July 14, 1953 Dicke Aug. 17, 1954 Farr July 12, 1955 

